Most manufactured articles are the result of several processing steps. Many manufactured articles are processed in lots with each lot containing multiple articles. The quality of the final articles in a lot is affected by each processing step. A defective or inferior article results wherever any of the processing steps varies beyond an acceptable limit. This is true whether the manufactured article is a semiconductor microcircuit, a piece of metal, printed money, a piece of glass, or any other article that undergoes one or more processing steps during its manufacture.
Once manufacturing of an article occurs for a sufficient period of time a set of measurable performance characteristics may be developed as a measure of the article's quality or "goodness." The performance characteristics for the article are typically measured at "final test" after all processing of the article or at a suitable point during the manufacture of the article. For example, in printing money, once the money is printed, the clarity of the printing provides a measure of quality of the printing. In semiconductor processing, electrically testing individual die on a completed semiconductor wafer determines the acceptability of each die and the yield for the wafer. In short, testing completed manufactured articles provides a measure of their acceptability.
Additionally, inadequacies or failures occurring at "final test" can generally be traced to a given process step in the manufacturing process. Prior systems for analyzing failures during testing use an analytical approach that looks at the number of unacceptable articles in the entire lot and the type of test failures to correlate the failures to a given process step during the manufacture of the articles. These prior systems for analyzing failures during testing of a lot of manufactured articles generally provide acceptable indications of the failure mechanism provided there is a single cause for the failures. If, however, the failures are the result of more than one failure mechanism, correlating the test failures to a given process step becomes more difficult.
One previously developed tool for correlating electrical failures in semiconductor die in a lot of wafers to a process step uses the yield of each wafer at multiprobe (MP) test with the sequential or spatial positions of the wafers at each process step in analyzing the failures at MP test. By knowing the failed test and the sequential or spatial processing of the wafers, this prior system can correlate the failures at MP test to a given processing step, e.g., ion implantation or anneal. Such a system works generally well provided the failures at MP test are the result of a single errant processing step. The Wafer Sleuth System developed by Sematech is such a failure analysis tool and is described in Sematech document entitled "Wafer Sleuth Implementation Guide" document 91060587A-ENG, dated Sep. 25, 1991.
Systems for analyzing and correlating failures at final test to manufacturing steps for other types of manufactured articles suffer from the same deficiencies as the semiconductor processing analysis system described above. Because these prior systems use only yield and sequential or spatial data in their analysis, they may fail to detect when multiple failure mechanisms result in unacceptable test results. Also, because prior systems for analyzing failures use purely numerical analysis, their outputs may require significant time to analyze and understand. Additionally, prior failure analysis systems are not affective when the lots processed contain a small number of articles, e.g., four pieces per lot, where the acceptance level would be 0%, 25%, 50%, 75% or 100%.